Image Compare: Color controls for slider

#### Is your feature request related to a problem? Please describe.

From testing report:
> Using the white slider on a white background make the slider hard to see. It would be great to allows the slider to be different colours or even have a slight shadow. So that it stands out.

#### Describe the solution you'd like

Add controls to sidebar to allow custom colors for slider or toggle light/dark

The issue does not identify the Image Compare implementation or its tests. Start by locating the Image Compare feature and its sidebar controls, then check how the slider is styled and whether existing tests cover it. Done means the slider can be made visible through the requested color or light/dark controls and the relevant tests pass.

Written by the indexing model from the issue text.